Found this little interesting bug when trying to mess with creating custom world presets. If you save a custom world preset in one preset slot and then, without changing anything, go and save a custom cave preset to the same preset slot (or vice versa, cave first then world) and then launch the world, you will get a world with 2 of the same shard types. It will be the world type of whatever preset you saved last, so if you save the world preset first then the cave preset, you'll get 2 cave worlds and vice versa.
These are from the same world:
And then these are from a different world generated the other way, with two overworld shards:
Just thought I'd shed some light on it for you guys
First save a world preset with any config setup (remember the preset slot number). Then, save a cave preset on the same save file to the same number preset slot as you did in step 1. Both the overworld preset number and cave preset number should be the same. If so, launch the game. It will get reset if any mods get toggled in the meantime. You should have two shards with the same world type that you can travel between with cave entrances. Can reverse the order of preset saving (caves first, then world) to reverse what shards you end up with.